Iano's backfill

The Comparitive Sustainability of Structural Materials

Materials Matter (Architecture Record, March 2011) compares the life cycle effects of wood, steel, and concrete construction. this article provides an informative discussion of life cycle analysis techniques and interesting comparisons of these three structural materials. (Caveat: The article is sponsored by a group affiliated with the forestry industry.)

Regarding comparing the embodied effects of various building materials, the current issue of Architecture Record (March 2011, p. 115) has an article comparing the life cycle effects of wood, steel, and concrete construction. As I mentioned in my talk, this article was sponsored by a group affiliated with the forestry industry. Nevertheless, I think the material is useful.

 

The article is also available online here: http://continuingeducation.construction.com/article.php?L=221&C=754 .

Guidelines for Hot-Dip Galvanizing

Galvanizing Tips, Modern Steel Construction, April 2004, discusses principals for successful hot-dip galvanizing of steel components, including:

• Symmetry: Balanced sectional shapes, such as H-sections, respond more uniformly to the thermal stresses of galvanizing and are less prone to distortion than unbalanced shapes such as C- or T-sections. Residual stresses from unbalanced forming or welding can have similar effects.
• Uniformity: Uniformity of thickness in section, of composition, and of surface character also reduce the distorting effects of thermal stresses, as well as promote a more consistent appearance of the galvanized finish.
• Cleanliness: Scale, rust, organic coatings, and other surface contaminants can interfere with the metallurgical reaction that bonds the molten zinc to the base steel.
• Access: Safe effective galvanizing depends on the free flow and draining of molten zinc running through the fabrication, on the free venting of steam, air, and vapors, and on access to appropriate lifting points.

The article also discusses relevant ASTM Standards, size limitations, and dipping techniques for oversize elements.

More Information
American Galvanizers Association

Emerging Technologies for Residential Construction

The Partnership for Advanced Technology in Housing (PATH) has announced its Top Ten Technologies for 2004, including:

• Frost Protected Shallow Foundations: Protect foundations against frost damage without the need for excavating below the frost line.
• Home Run Plumbing Systems: Feeds flexible piping directly to one fixture from the central manifold.
• Engineered Panelized Systems: Prefabricated panels form a structural envelope that reduces or eliminates the need for on-site framing and can be assembled swiftly.
• HVAC Optimization: To maximize efficiency and comfort, use properly sized equipment, and place HVAC equipment inside the conditioned space.
• Tankless Water Heaters: Provide hot water on demand, reducing or eliminating standby loss.
• Shared (Community) Waste Water treatment: A single drainfield/treatment area connected to each house's individual septic tank.
• Air Admittance Vents: Pressure-activated, one-way plumbing valves, eliminating the need for conventional pipe venting and roof penetrations.
• Low Impact Development: LID techniques can offer a cost-effective way to address stormwater management through site design and "Best Management Practices".
• Integrated Steel/Wood Combination Framing: Combinations of wood and steel framing within the overall building shell use the best attributes and cost benefits of each.
• Pre-cast Concrete Panels (Walls and Foundation): Made under quality-controlled factory conditions, pre-cast concrete panels are ready in a fraction of the time needed for a poured foundation.

Several of the Top 10 may be of particular interest to readers of Fundamentals of Building Construction. See the PATH page links below for additional technical background, information on regulatory acceptance, and links to additional resources and manufacturers related to these systems:

Engineered panelized systems discusses a variety of innovative insulated panel systems including composites of rigid foam/light gauge steel framing, rigid foam/concrete, and rigid foam/fiberglass/PVC. For a brief introduction to prefabricated panel systems, see also page 100 of the textbook.

Shallow frost-protected foundations are also discussed and illustrated on page 65 of the textbook. In these systems, rigid insulation installed below grade protects against soil freezing and allows shallow footings to be constructed above the normal frost line.

Combination steel/wood framing combines light gauge steel studs and joists with wood framing members such as sills, band joists, and plates. These systems offer the advantages of steel framing (light weight, recyclability, dimensional stability, resistance to moisture, etc.) while relying on traditional wood light framing connection methods and tools (nail guns, staples, etc.). Both loadbearing and nonloadbearing systems are feasible.

Pre-cast concrete foundation panels are insulated, loadbearing, reinforced concrete panels that according to PATH, allow erection of residential basements in less than one day. Panels come with concrete "studs" and metal or preservative-treated furring for attachment of finishes. [Author's note: Waterproofing of joints between panels appears to rely on conventional joint sealants, a potential long-term vulnerability.]

What is PATH?
PATH describes itself as

a voluntary partnership in which leaders of the homebuilding, product manufacturing, insurance, and financial industries join forces with representatives of Federal agencies concerned with housing. By working together, PATH partners improve the quality and affordability of today's new and existing homes, strengthen the technology infrastructure of the United States, and help create the next generation of American housing.

Its Top 10 Technologies are intended to represent "practical, easy-to-use building technologies that are on the edge of industry acceptance and are quickly gaining importance."

Designing with Metals: Dissimilar Metals and The Galvanic Series

The galvanic series is a list of metals arranged in order of their relative electrical potential. A simple version of the galvanic series is shown in Figure 16.55, page 604 of the textbook. When two metals are in contact in the presence of moisture, their locations within the series indicate the risk of corrosion due to the flow of electric current between them. The closer the two metals on the list, the less the difference in electrical potential, and the less the risk of corrosion. The further apart the two materials on the list, the greater the risk of corrosion. The following are some guidelines for working with dissimilar metals and interpreting the galvanic series.

Avoid contact between metals far apart on the galvanic series.
Virtually every student of building technology is taught this most basic fact about the galvanic series. However, when presented in its usual list form, the galvanic series provides only minimal guidance on judging the relative differences between metals and evaluating their potential incompatibility. A more complete picture of the compatibility of metals can be constructed when numeric values for the metals' electrical potential are attached to the list as well.

This chart lists common architectural metals along with their ranges of relative electrical potential. As with the galvanic series, metals are arranged in order of increasing potential, but in this case, the relative differences between various metal types are more readily apparent.

For example, in the chart above consider the aluminum bronze alloy group and the next metals listed directly above and below. We can see that between the aluminum bronze alloys and the brass alloys directly below (naval, red, and yellow brasses), the relative difference between these metals is small. On the other hand, the difference between aluminum bronzes and mild steel, cast iron, and wrought iron directly above is many times greater. In fact, one must read down the list nine or more metals below aluminum bronze before the electrical potential difference is comparable to moving up only to the first metals above.

With quantified potential differences between metals, the galvanic series can also be used to estimate the compatibility of different metals under varying environmental conditions using the following rules of thumb:

• In coastal, very high humidity, or other harsh environments, galvanic metal pairs should be limited to those with a potential difference no greater than 0.15 volts.
• In moderate environments, metal pairs should have a potential difference no greater than 0.25 volts.
• In environments with controlled humidity and temperature, potential differences as great as 0.50 volts may be acceptable.

For example, consider again the aluminum bronze alloy group. In a harsh environment, the designer may opt to limit metals to be used in contact with this alloy group to other bronze alloys, brasses of various types, copper, tin, and 400 series stainless steel. On the other hand, in a controlled environment, aluminum bronze might safely be combined with any other metal listed on the chart, with the exception of zinc and galvanized steel.

Based on these rules of thumb, metals listed in the chart have been color-coded into groups that fall within potential difference ranges of roughly 0.20 volts. Metals within each of these groups may be considered least corrosion prone when used together in normal architectural conditions.

Avoid smaller anodes in contact with larger cathodes.
On the chart above, the more negative end of the potential scale is noted as anodic or active, and the more positive end of the scale as cathodic or passive. When different metals react galvanically, an exchange of electrons takes place between the two metals, with electrons flowing from the metal with greater negative potential (the anode) to the metal with lesser negative potential (the cathode). For example, if aluminum bronze and a 300 series stainless steel are used together, the aluminum bronze has a greater negative potential and will act as the anode, donating electrons to the less negative stainless steel, the cathode. On the other hand, if aluminum bronze is used with mild steel, mild steel has a greater negative potential and will act as the anode, donating electrons to the aluminum bronze, which in this case acts as the cathode. (A note on terminology: Literature on galvanic reactions often refers to cathodic metals as noble. These two terms are synonymous.)

To a chemist, the anode’s release of electrons is termed oxidation. In laymen’s terms this is known as corrosion. In other words, with any galvanic pair of metals, the anode corrodes as the galvanic reaction takes place. Controlling the rate of corrosion of the anodic metal is an important consideration in working with galvanic metal pairs. After consideration of the electrical potential difference between the two metals, the next most important factor governing the rate of corrosion of the anode is the relative surface area of the anode in comparision to the cathode. The smaller the surface area of the anode in relation to the cathode, the more concentrated the flow of electrons, and the faster the rate of corrosion. The larger the anode's surface area in relation the cathode, the more spread out the flow of electrons, and the less the corrosion. This principal, called the area ratio, often has important architectural implications.

For example, consider a sheet metal copper roof fastened with Type 304 stainless steel screws. The potential difference between the two metals is in the range 0.2 to 0.3 volts, so some corrosion effects may be expected under exterior conditions. In this galvanic pair, copper has a higher negative potential and will act as the anode and stainless steel will act as the cathode. However, since the surface area of the anode (the copper roof metal) is large in comparison to the surface area of the cathode (the stainless steel fasteners ), the corrosive effect on the copper is distributed over a relatively large area and greatly mitigated. In this case little if any long-term negative effect is anticipated. In fact, in practice, stainless steel screws are an accepted method of attachment for copper roofing.

As a counter example, consider a stainless steel sheet metal roof fastened with copper nails. In this case, the surface area of the anode (the copper fasteners) is very small in relation to the surface area of the cathode (the stainless steel roof metal), the flow of electrons from the anodes is highly concentrated, and rapid corrosion of the fasteners is expected.

In fact, the rate of corrosion of the anode in a galvanic metal pair is directly related to the numeric area ratio of the two metals. That is, if the surface area ratio of cathode to anode is doubled, the rate of corrosion of the anode is also doubled. Likewise, if the area ratio is halved, the rate of corrosion of the anode is halved.

Avoid Fasteners Acting as Anodes
The previous examples illustrate an important guideline, that fasteners should generally be selected to avoid taking on the role of anode in a galvanic reaction. Due to their normally small surface area in relation to the materials being fastened, such fasteners will be at risk of rapid corrosion. Thus when fastener and base metal differ, the fastener metal should be selected to be cathodic in relation to the base metal. When two dissimilar metals are joined with a third fastener, the fastener should be cathodic in relation to at least one of the other metals, so that it does not take on the role of anode in a galvanic reaction between the three.

There are many finer points regarding the selection of metal fasteners in relation to metals being joined, the building environment, and the particular application. This topic will be addressed more fully in a later article in this Designing with Metals series.

Avoid Rainwater Runoff From Cathode To Anode
Consider a sheet metal copper roof with a galvanized steel gutter. Rainwater flowing over the roof metal will pick up copper in solution and carry this dissolved metal into the gutter. When the dissolved copper and galvanized steel come into contact, these metals will react as a galvanic pair. Since the galvanized steel is anodic to copper, the gutter will be corroded. Alternatively, consider a galvanized steel roof and a copper gutter. In this case, dissolved zinc (from the galvanized coating on the steel) is carried into the copper gutter, where the zinc will act as the anode in the galvanic pair. In this case the copper gutter acts as the cathode and is not threatened with corrosion.

As a general rule with metal roof and wall systems, care should be taken that rainwater does not flow from metal surfaces that are relatively cathodic to others that are relatively anodic.

Treat Plated Metals According To Their Plating
When using galvanic series charts with plated metals, read from the position of the plating, not the base metal. For example, a cadmium plated mild steel fastener reacts according to the electrical potential of cadmium, not mild steel. Galvanized steel (a zinc metal coating on steel) reacts according to the electrical potential of zinc. Lead coated copper sheet will react according to the electrical potential of lead, not copper. Etc.

Understand Your Project Particulars
In practice, there are additional considerations that can influence the severity of the galvanic reaction between metals. For example, coastal environments tend to produce salt-laden precipitation that can significantly accelerate the reaction between galvanic metal pairs in comparison to non-coastal areas. Urban and industrial environments, with their relatively high concentrations of air pollutants, produce precipitation that is more acidic and conducive to corrosion than precipitation further from such areas. Perhaps less obvious is the potential for accelerated corrosion in some agricultural environments, where certain fertilizers are a known source of corrosive air pollutants.

The electrical potential of metals may vary depending on the medium in which the galvanic reaction takes place. In fact, most galvanic series, including the chart in this article, are based on the electrical potential of metals when immersed in flowing sea water. The designer should keep in mind that metals buried in soil, exposed to highly corrosive industrial solutions, or otherwise exposed to atypical environments may react quite differently from what is predicted by the standard galvanic series.

As one example, consider a 300 series stainless steel angle buried in mud. Due to a lack of free oxygen in such a soil condition, the stainless steel may not be able to maintain the passive layer that normally protects the underlying metal from corrosion. In this condition, the stainless steel surface can become more electrochemically active and assume a location in the galvanic series close to mild steel, a change in its electrical potential of approximately -0.5 volts. (For a discussion of stainless steel alloys and its active and passive conditions, see the related article Stainless Steel and Corrosion Resistance.) As another example, curiously, zinc may become cathodic to iron when immersed in hot tap water.

The particulars of a metal detail or assembly can also influence the rate of corrosion. For example, low-slope metal roofs, in which standing water can accumulate, may exhibit higher rates of corrosion in comparison to steeper roofs that shed water more rapidly and therefore remain dryer. Details that capture and trap water can also lead to accelerated corrosion in localized areas.

Approach Insulating Strategies Cautiously
One strategy for mitigating corrosion between metals is to insulate the metals from each other so that the electrochemical reaction can not take place. While this strategy is theoretically sound, in practice it must be approached with caution.

For example, consider a copper roof fastened with galvanized steel anchor clips. This is not a recommended assembly since the galvanized steel is anodic to the copper. Given the relatively small surface area of anchor clips in relation to roof metal, the anchor clips are expected to corrode rapidly. One way to attempt to overcome this problem might be to apply a non-conductive coating, such as asphalt mastic, to the anchor clips, thereby preventing electrical contact between the two metals. However, in practice, it must be assumed that some gaps will appear in the coating, either due to imperfect application or due to wear and tear over time as the metals expand and contract. In either case, where gaps occur, the galvanic reaction will proceed. Furthermore, given the even smaller area ratio between just these exposed areas on the anodes and the larger area of cathodic roof metal, the reaction will proceed in these areas at an even more accelerated rate than it would have otherwise.

In other words, applying insulating coatings to only the anode in a galvanic pair is strongly discouraged, as it may actually increase the risk of corrosion. When insulating coatings are used to prevent electrical conduction between galvanic pairs, the cathode should always be coated, whether the anode is coated or not.

As another example, consider a black rubber washer between a fastener and roof of different metals. If the washer contains a high percentage of carbon black (used to color the washer), it may be sufficiently electrically conductive to allow the galvanic reaction to proceed between the two metals. Furthermore, even with an insulating washer, the two metals remain in contact where the fastener penetrates the roof sheet, and the galvanic reaction can still proceed through this juncture.

Conclusion
The galvanic series is a powerful tool for evaluating the potential risk of corrosion between metals. However, to be used effectively, it must be applied knowledgeably and with consideration of factors that may influence the risk of corrosion between dissimilar metals. Wherever possible, past experience and local knowledge should be included in these considerations.

This is the second article in an occasional series on architectural metals.
Stainless Steel and Corrosion Resistance
Dissimilar Metals And The Galvanic Series (this article)
Next: Selecting Metallic Fasteners

For more information:
ASTM International's ASTM G 82 Standard Guide for Development and Use of a Galvanic Series for Predicting Galvanic Corrosion Performance includes the galvanic series, guidelines for its interpretation, and information on the deriviation of electrical potential values.
Corrosion Doctors and CorrosionSource.com web sites provide extensive information on many aspects of the corrosion of metals. See for example The Galvanic Series, Introduction To Design and Corrosion, Prevention of Galvanic Corrosion By Design, and Galvanic Compatibility
Department of Defense, Military Specification Finishes for Ground Based Electronic Equipment, MIL-F-14072D(ER) provides some useful guidelines and technical details for working with the galvanic series.

Metal Framing For Residential Structures

Framing With Light-Gauge Steel, Journal of Light Construction November 2003, provides a clear discussion of:

• Metal framing components and terminology
• Tools for cutting and fastening metal framing
• Standard details for residential construction
• And more...

This seven-page, well illustrated article is a good read for anyone trying to increase their familiarity with this system.

More Information:
Chapter 12 of the textbook covers light gauge steel frame construction materials and methods.

Designing With Metals: Stainless Steel and Corrosion Resistance

The textbook, page 459, describes stainless steel as a steel alloy containing chromium and nickel, and being highly resistant to corrosion. Stainless steel is commonly used in architectural applications such as metal roofing, wall panels, railings, sanitary surfaces, flashing, shelf angles, lintels, masonry veneer anchors, and hardware and fasteners exposed to moisture. Stainless steel is 100 percent recyclable, and stainless steel made today typically contains 65 to 80 percent recycled content.

What is Stainless Steel?
Steel containing approximately 10 percent or more chromium qualifies as stainless. The chromium at the surface of the metal combines with oxygen from the atmosphere to form a thin, clear oxide film. Once formed, this oxide layer itself does not react with most corrosive elements and in effect forms a tight, protective seal around the metal. Under normal conditions, even if the oxide layer is scratched or damaged, the oxide layer will re-form, essentially healing itself and maintaining protection of the underlying metal. In the technical literature, this oxide layer is said to create a passive barrier on the surface of the steel.

Shown from front to back are flashing samples made from stainless steel, copper, and galvanized steel. Compared to copper, stainless steel is harder and stiffer, in these respects making it more difficult to work with as a flashing material. Copper has a more distinctive color and produces runoff that may cause staining. Galvanized steel is the least expensive of the three, but also is not as long lasting. (Hues in this digital image have been slightly intensified to accentuate the differences in color between the three metals.)

What Are the Common Types of Stainless Steel?
Different stainless steel alloys are distinguished by the varying amounts of chromium and other metals added to the steel. Within the range of common stainless steel alloys, the higher the percentages of chromium and nickel, generally speaking the more corrosion-resistant the alloy. In architectural applications, the most commonly specified stainless steel alloy is Type 304. Type 304 stainless steel has 18 to 20 percent chromium, 8 to 12 percent nickel, and smaller amounts of other elements. Chromium provides the base level of corrosion resistance. Nickel adds additional corrosion resistance and improves the ductility of the metal. (Steel alloyed with just chromium tends to be hard and brittle.)

In highly corrosive environments Type 316 stainless steel is recommended. This alloy differs from Type 304 in the addition of 2 to 3 percent molybdenum, and an increase in the percentage of nickel. The result is greater resistance to chlorides and other corrosives. This alloy is particularly noted for its resistance to a form of corrosion called pitting that is common in marine environments. Type 316 stainless steel is also more expensive than Type 304.

Where stainless steel requires heavy welding, variations on these two alloys may be used. At the high temperatures of welding, carbon in the steel combines chemically with the chromium, rendering the chromium unavailable for the formation of the protective oxide coating. Without the ability to form this coating, corrosion-prone areas form in the areas surrounding the weld. In Type 304L and Type 316L stainless steel, the carbon content is reduced from a maximum of 0.08 percent to less than 0.03 percent. The reduction in carbon ensures that chromium remains available for formation of the oxide coating. As a side effect, the yield strength of these alloys is also reduced, from approximately 30,000 psi (205 MPa) to 25,000 psi (170 MPa).

The most common stainless steels are also sometimes referred to as austenitic. This term describes the crystalline metal structure of these alloys (face-centered cubic). The austenitic stainless steels include the 300 series alloys as well as some less common 200 series, lower-nickel, alloys. In comparison to other alloys, austenitic stainless steels are higher in chromium and nickel, low in carbon, and highly corrosion resistant. They have rates of thermal expansion 30 to 50 percent greater than normal carbon steels. They are also distinguished by being non-magnetic, although some can exhibit mild magnetic properties after cold working.

One limitation of austenitic stainless steels is that they cannot be hardened by heat treatment. Type 410 stainless steel is a martensitic alloy containing roughly 12 to 14 percent chromium, little or no nickel, and up to 0.15 percent carbon. Type 410 stainless steel may be hardened, though it also has less corrosion resistance and is less ductile in comparison with the 300 series high nickel alloys. Case hardened Type 410 stainless steel may be used, for example, in the manufacture of self-drilling or self-tapping screws for fastening to steel or concrete, where Type 300 series alloys would lack sufficient hardness to cut through these dense materials.

Some architectural stainless steel may also be referred to as 18-8 stainless steel. This term refers to Type 304 and a few other closely related alloys, all of which have approximately 18 percent chromium and 8 percent nickel, and all of which share similar levels of corrosion resistance and other physical properties. In many cases the term 18-8 is used interchangeably with Type 304.

Two stainless steel fasteners from a manufacturer's catalog listing are shown. Note that the self-drilling screw for fastening into wood is made from 18-8 (Type 304) stainless steel, while the fastener designed to cut threads in much harder steel is made from hardened Type 410 stainless steel.

What Is the Difference Between "Active" and "Passive" Stainless Steel on the Galvanic Series?
The galvanic series is a list of metals arranged in order of their electrical potential. A simple version of the galvanic series is shown in Figure 16.55, page 604, of the text. When two metals are in contact and in the presence of moisture, their relative locations within the series indicate the risk of corrosion due to the flow of electric current between them. The closer the two metals on the list, the less the difference in electrical potential, and the less the risk of corrosion; the further apart the two materials on the list, the greater the risk of corrosion.

Many galvanic series lists show stainless steel in two locations, one for passive stainless steel and another for active. These terms refer to the presence or absence of the protective oxide coating that normally forms on the surface of the stainless steel, as discussed above. Under any normal circumstance, stainless steel used in architectural applications will exhibit this oxide coating and thus its galvanic properties should be referenced from its passive location within the galvanic series. While it is possible for the surface of stainless steel to become active under certain conditions, such circumstances are not common to architectural applications, and references to active stainless steel on the galvanic series should normally be ignored.

This is the first article in an occasional series on desining with architectural metals.
Next: Dissimilar Metals And The Galvanic Series

More information:
The textbook discusses architectural uses of metals on pages 458 - 460.
Why Is Stainless Steel Stainless? provides an easy to understand explanation of stainless steel basics, and links to additional informative sites.
Corrosion, Stainless Steel is a clear and technically detailed account of corrosion mechanisms in stainless steel.
Prevention of galvanic corrosion by design provides a brief summary of design strategies for avoiding galvanic corrosion between dissimilar metals, and lists links to related information.
Stainless Steel Information Center offers extensive reference information on the properties and uses of stainless steel.
Architectural Metals, by L. William Zahner (John Wiley & Sons, Inc., 1995), provides in-depth technical and design information on stainless steel and other architectural metals.

12 - Light Gauge Steel Framing Links

This article contains external links to resources on the Web relevant to Chapter 12  Light Gauge Steel Framing.

American Galvanizers Association

Industry trade group providing technical information on galvanizing and other zinc coatings over steel.

Copper.Org

Copper Development Association's technical resources center. See especially their Design Handbook.

GalvInfo Center

Technical notes on galvanzing & related zinc coatings

Light Steel Framing Manual

MetalFraming.org's online technical manual.

SSMA Technical Library

Technical resources from the Steel Stud Manufacturers Association

USG Drywall/Steel Framed Systems

Systems catalog, details, specifications